Electrodepositing iron



United 2,714,080 Patented July 26, 1955 ELECTRODEPOSITIN G IRON WalterR. Meyer, Hamden, Comm, assignor to Enthone, Incorporated, New Haven,Conn., a corporation of Connecticut No Drawing. Application January 26,1953, Serial No. 333,358

6 Claims. (Cl. 204-48) This invention relates to the art of plating ironfrom 1.

alkaline plating baths and to alkaline compositions of matter suitablefor preparing iron plating solutions and/or replenishing partiallydepleted iron plating baths. The invention also relates to theiron-plated articles and electro-formed articles prepared by saidplating process.

Electrodeposition of metals from alkaline solutions offers numerousadvantages not to be found in the use of acid baths. The acid fumes in aplating room employing acid baths have created both health hazards andcorrosion hazards. Many articles are attacked when immersed in an acidplating bath. Many articles have had such shapes that there have beencorrosion problems at the low current density areas of the articlesplated from acid baths. Platings which appeared satisfactory initiallyhave sometimes developed blisters and other imperfections attributableto the use of an acid bath. For these and other reasons, electrochemistshave sought to develop suitable alkaline plating processes for eachmetal employed significantly in the electrodeposition industries. As aresult there has been widespread practice of alkaline plating of metalssuch as gold, silver, copper, zinc, cadmium, tin and lead. Such alkalineplating baths have generally had more uniform plate distribution andthrowing power than acid baths.

For many decades there has been a continuing demand for an alkalineplating process for iron. There have been proposals for using cyanidecomplexes of iron in order to plate iron at very slow rates at loWcurrent efiiciencies and at very small current densities,

but such proposals have not been of commercial interest.

Hence, the ferrocyanide techniques have not been mentioned in recentdescriptions of industrial plating of iron.

Recent improvements relating to the electrochemistry of iron underalkaline conditions have concerned, not

the deposition of a commercially useful iron plating, but otherpurposes. Iron powder has been prepared electrolytically under alkalineconditions. It has been proposed that a thick, viscous paste of alkalisand ferric hydroxide be subjected to electrolysis between closely spacedelectrodes, but this process did not gain wide acceptance, partlybecause thick pastes are more diificult to use than liquid electrolytes.Iron oxide cathodes cannot be readily reduced by electrolysis of simplealkaline solutions, but a process was proposed for accomplishing such areduction using both alkali hydroxides and alkali sulfides in theelectrolyte. Although industrial use of electro-plating of iron fromacid baths has been increasing and although there have been numerousrefinements in that art, although there has been a continuing andincreasing demand for an alkaline iron plating method, and althoughthere have been divers inventions concerning the alkalineelectrochemistry of iron, still no commercially operable process for thealkaline plating of iron was heretofore developed.

It is an object of the present invention provide a commercially usefuliron plating electrodeposited from an alkaline solution, which platingis satisfactorily adherent to its base, and which has the other goodcharacteristics of an industrially useful metallic plating. In statingthat an object is to provide a commercially useful iron plating, thereare included requirements such as: that the plating be generally freefrom pits, pinholes, blisters and related imperfections; that theplating be free from excessive stress or brittleness; that the platinghave structural strength appropriate in electroformed articles; that theprocess provides uniform plate distribution and good throwing power; andthat the process provides relatively high current efficiencies. Suchobjectives are attained by means of a novel plating bath, a novelcomposition employed in preparing said plating bath, by a novel methodof electroplating with said bath, and/or by other techniques and waysdescribed hereinafter.

. Further objects of the invention are: to eliminate the corrosionproblems incident to acid plating of iron; to eliminate the corrosivefumes incident to the use of an acid bath; to form consistently smoothdeposits of iron in an economical manner and with relatively highcurrent efl iciencies; to form mechanically strong iron articles such aslithograph plates by electrodeposition of iron upon removable forms; toproduce bright iron plating; and to conduct the iron plating rapidly,conveniently, with high current efficiency, and especially withreasonably uniform plate distribution, that is, with good throwingpower.

A feature of the invention is the use of a plating bath complying withseven requirements, each of the seven requirements being absolutelynecessary to the combination. The plating bath is aqueous and generallythe molar concentration of water is greater than the concentration ofany other component. An alkali such as sodium hydroxide must be presentin a significant concentration in excess of 0.5 molar but less than 5molar. An organic amine such as triethanolamine must be utilized in asignificant concentration in excess of 0.5 molar but less than 5 molar.An organic sequestering agent such as the sodium salt ofethylenediaminetetraacetic acid cooperates with the organic amine inpreventing the precipitation of the iron, the ratio of the molarconcentration of the sequestering agent to the molar concentration ofiron being from 0.5 to 1.5. During the plating operation, the iron mustbe present predominantly as ferric iron, and if a bath is preparedinitially using ferrous salts, an opportunity should be given forconverting most of the ferrous iron to ferric iron before extensivecommercial plating therewith. Exposure to the air rapidly convertsstrongly alkaline ferrous solutions to predominantly ferric solutions.The concentration of the iron must be more than 0.02 molar and less thanapproximately 0.5 molar. Viscosity modifiers, wetting agents, surfacetension modifiers, salts, and other additives for a plating bath may bepresent if desired. However, the alkalinity of the solution must begreat enough to provide a pH greater than 11.

In even slightly alkaline solutions, ferric hydroxide is insoluble, asindicated by a solubility product of 4 l0* Ferric hydroxide differsgreatly from aluminum hydroxide in that even in strongly alkalinesolutions, only small amounts of ferric hydroxide dissolve. However, thecommercial plating baths of the present invention employ a minimum of0.02 molar concentrations of iron, desirably a concentration ofapproximately 0.3 molar iron and up to 0.5 molar iron. That is, thecommercial plating baths of the present invention have an ironconcentration within the range of from about 1 to 28 and preferablyabout grams of iron per liter of plating bath. Thus the plating bathscontain exceptionally large concentrations of iron in alkalinesolutions. a V

In the development of the present invention, it was found that anaqueous solution of triethanolamine formed a blue-green precipitate withiron compounds if the pH was below 11, but did form a green solutionabove pH ll. Thus it was discovered that the combination oftriethanolamine and sodium hydroxide dissolved greater amounts of ironthan either a solution of sodium hydroxide or a solution oftriethanolamine. The amount of iron thus dissolved in triethanolamineand sodium hydroxide mixtures was much greater when ferric salts wereemployed instead of ferrous salts. A solution containing 1 Mtriethanolamine and 2.5 M sodium hydroxide dissolved 20 grams per literof iron as ferric chloride, but no satisfactory method of commerciallyelectroplating with such a solution was found notwithstanding itsexceptionally high concentration of iron.

The sodium salt of ethylenediaminetetraacetic acid (hereinaftersometimes referred to as EDTA acid) formed reddish brown precipitateswith moderate concentrations of iron under alkaline conditions. In thedevelopment of the present invention, it was found that the combinationof triethanolamine and the sodium salt of EDTA acid was very much morethan additive, and that the combination made it possible to conductsatisfactory electroplating with the plating baths complying with theenumerated requirements. Even with the remarkable combination of sodiumhydroxide, triethanolamine and the salt of EDTA acid, the significantamounts of iron could not be kept in solution at mildly alkalineconditions, the traces of suspended precipitate imparting a red color tothe solution below pH 11. From a practical standpoint it was found bestto maintain the alkalinity and triethanolamine concentration high enoughto preserve the green color and to prevent the brownish or redcoloration indicative of low alkalinity, using the solution as aninherent indicator of pH. The measurement of pH at high alkalinity isalfected by such factors as salt concentration, temperature and othervariables which result in less accuracy than in nearly neutralsolutions. For any combination of ingredients there is a criticalalkalinity which must be exceeded to achieve the necessary solubility ofiron and/or satisfactory plating, which critical point is above pH 11 bysome conventional measuring method, and which is also detectable by atransition having some similarity to the red to green transformationdescribed.

Heretofore metals have ordinarily been electrodeposited from a loweroxidation state, but according to the present invention, advantageousresults are achieved by deposition of iron from a ferric-type bath. Theiron kept in solution by the combination of EDTA salt, amine, and alkaliis predominantly in the ferric condition. Although ferrous iron mayexist near the cathode, or even in the body of the solution, still formost practical purposes the solution can be treated as if it were aferric solution. An analysis of the solution using titanous chlorideindicated that the quantity of ferrous iron present was less than couldbe detected by the analytical procedure. Even if the presence of ferrousiron was indicated by some other analysis, still the plating solutioncould be treated as if all iron was present as ferric iron, thusachieving very important advantages. For example, the plating solutionscan be agitated by compressed air. None of the precautions required forpreserving sensitive ferrous solutions need be observed, thus greatlysimplifying commercial iron plating practice. In preparing the platingbath, ferric salts are desirably employed. If ferrous salts are added toan otherwise complete bath, they are not dissolved immediately, but onlyafter oxidation thereof to the ferric iron. Strongly alkaline solutionsof ferrous compounds are readily oxidized to ferric compounds byexposure to air.

In order that those skilled in the art may better understand how thepresent invention may be practiced, the following examples are given byway of illustration and not by way of limitation.

EXAMPLE 1 A plating bath was prepared consisting of sodium hydroxide,triethanolamine, the sodium salt of ethylene diaminetetraacetic (EDTA)acid, a ferric salt, and water, as follows:

NaOH g (HOCH2CH2)3N g 231 ((NaO2CCH2)zNCH2)2 g 132 Fe+++ g 20 H2O,q s l1 A sheet of copper was made the cathode of a plating cell having sheetsteel anodes, and a direct current was passed through said plating bathat a current density of 40 amperes per square foot. The temperature ofthe bath was F. A bright coating resulted. The current efiiciency wasfound to be 88%.

EXAMPLE 2 The effect of varying amounts of alkali was measured bydetermining the current efficiency at 158 F. and 40 amperes per squarefoot. In each case the bath contained: triethanolarnine, the sodium saltof ethylenediaminetetraacetic acid, a ferric salt and Water as follows:

(HOCH2CH2)3N g 154 ((NaOzCCHzOzNCHzh g 132 Fe+++ g 20 H2O, q. s. to l lThe variable in the series of experiments was the concentration ofalkali, as shown in the following table.

From such data it was concluded that the optimum concentration of alkaliwas approximately 2.5 molar. Separate tests demonstrated that a solutioncontaining 10 molar sodium hydroxide did not dissolve significantamounts of the sodium salt of EDTA acid. Very low current efiiciencieswere observed in plating with 7.5 molar alkali. By such methods it wasestablished that the alkali concentration should be from 0.5 to 5 molar.

EXAMPLE 3 The molar ratios of sequestering agent to amine and the molarratios of sequestering agent to iron were investigated. In each test thefollowing conditions and bath components were maintained.

NaOH g 100 Fe+++ g 20 H2O,q.s -l 1 T F 180 C. D amp/sq. ft 40 In thefollowing table, A refers to the molar ratio of sequestering agent toamine, B refers to the molar ratio of sequestering agent to iron, Crefers to the grams per 5 liter of triethanolarnine, D refers to thegrams per liter of the sodium salt of EDTA acid, and E refers to currentBy tests such as outlined above it was established that the molar ratioof the sequestering agent to iron must be within the range from 0.5 to1.5, but that the ratio of sequestering agent to amine, which wouldrarely if ever exceed 1.0, was not critical if the amine concentrationexceeded 0.5 molar but was less than 5 molar. The molar ratio of amineto iron would in any case exceed 1.0.

EXAMPLE 4 A plating bath was prepared consisting of sodium hydroxide,triethanolarnine, the sodium salt of ethylenediaminetetraacetic acid, aferric salt and water as follows:

NaOH g 120 (HOCHZCH2)3N g 193 ((NaOzCCH2)2NCH2)2 g 165 Fe+++ g 25 H2O,q. s. l I Said solution was employed under conditions including:

T F 158 C. D a1np/sq. ft 40 The average current efficiency in duplicateruns was 44%. The same current efiiciency was measured when the alkaliconcentration was increased to 150 grams per liter.

Such solutions were more viscous than those containing the optimumconcentration of 20 grams per liter of ferric iron. It was establishedthat 1 molar iron was be yond the upper workable limit, which wasapproximately 0.5 molar. Commercial plating would not be attempted belowthe lower limit of 0.02 molar ferric iron, or approximately 1 gram perliter.

On the basis of experiments such as outlined, the bath compositionrequirements using said components (i. e. sodium hydroxide,triethanolamine, the sodium salt of ethylenediaminetetraacetic acid, aferric salt and water) were set as follows, expressed as molarities:

Min Opt. Max.

1 Not critical.

Such concentrations are equivalently expressed in grams per liter asfollows:

N 80H 100 200 (HOCH2CH2)3N. 75 149 745 ((NaOzCCH2)2NCH2 1 132 280 Fe .La 1 20 28 EXAMPLE 5 amine, the sodium salt ofetliylenediaminetetraacetic acid, a ferric salt and water, as follows:

NaOH g 100 (HOCH2CH2)3N g 154 ((NaO2CCH2)2NCH2)3 g 132 Fe+++ g 20 H2O, qs l 1 The results are summarized in Table 2.

Table 2 Current Efiieiency, Percent Condition of Surface Run #1 Run #2Average These data show that temperatures above F. permit higher currentefliciencies. A temperature of F. was selected as the optimum.

EXAMPLE 6 Although elevated temperatures are advantageous, commerciallysatisfactory plating can be performed at room temperature. A bath wasprepared consisting of sodium hydroxide, triethanolamine, the sodiumsalt of ethylenediaminetetraacetic acid, a ferric salt and water, asfollows:

NaOH g 30 (HOCH2CH2)3N g 116 ((NaO2CCH2)2NCH2)2 g 99 Fe+++ g 10 H2O, q sl 1 A bright plate was obtained at room temperature employing currentdensities up to 230 amperes per square foot of cathode.

EXAMPLE 7 The bath of Example 4 was employed for ten minutes per test at158 F. to determine the effect of current density upon currentefiiciency.

T able 3 Current Efficiency, Percent Current Density, amp/sq. it.

Run #1 Run #2 Average The same solution was employed at 180 F. in aseries of tests uniformly using 40 ampere-minutes of current with thefollowing results.

Current Efficiency,

Current Density, amp/sq. ft.

Percent A plating bath might be prepared consisting of potassiumhydroxide, piperidine, the sodium salt of EDTA acid, water and ferricchloride, as follows:

KOH g 114 HN(CH2CH2)2NH g 220 ((NaO2CCH2)2NCI-I2)2 g 132 FeCls g 60'HzO, q. s. to"; l 1

Such a plating bath might be used for an electroplating operation at 100F. using a current density of 40 amperes per square foot.

EXAMPLE 9 A plating bath might be prepared using:

KOH g 114 (H3CCHOHCH2)3N g 300 ((NaOaCCI-IzhNCl-Izh g 132 Fe(NOg)z g 80H2O, q. s. to l 1 Such a plating bath, consisting of potassiumhydroxide, triisopropanolamine, the sodium salt of EDTA acid, water, andferric nitrate, might be utilized for electroplating iron onto a basemetal such as copper, using a current density of approximately 300amperes per square foot and a temperature of 180 F.

EXAMPLE A plating bath might be prepared consisting of lithiumhydroxide, triethanolamine, dimethyl formamide, water and ferricnitrate, as follows:

LiOH g 48 (HOCH2CH2)3N g 500 (H3C)2NHCO g Fe(NO3)3 g 80 H2O, q. s. to l1 Such a bath might be used at room temperature for electroplating irononto a copper base at a current density of 10 amperes per square foot.

EXAMPLE 11 A plating bath might be prepared containing:

NaOH g (HOCH2CH2)3N g 300 NHO2C(CHOH)4CH2OH g 22 Fe(NO3)3 g 20 H2O, q.s. to l 1 Such a plating bath, containing sodium hydroxide,triethanolamine, sodium gluconate, water and ferric nitrate, might beused at room temperature for electroplating iron onto a graphite-coatedwax mold at a current density of 20 amperes per square foot.

EXAMPLE 12 The plating bath of Example 5 was used for the galvanicplating of iron onto aluminum. The aluminum sheet was well cleaned andthen immersed for 10 seconds in said plating solution. After rinsingwith water, the iron coating was sufficiently adherent to be suitable asa base for a subsequently applied electroplating of nickel.

EXAMPLE l3 Controlled experiments were run to determine the desirabilityof stirring the plating bath. It was found thatsuch agitation of theplating bath, although not absolutely necessary for commercially usefulplatings, was generally desirable.

EXAMPLE 14 Controlled experiments were run to determine the magnitude ofloss of weight of iron anodes during the electrolysis when the bath didand did not contain significant amounts of chloride ion. It was foundthat the rate of anode dissolution was much more rapid when the halideion was present.

EXAMPLE 1s After any of the plating baths has become partially depletedin its iron content, the bath may be regenerated by the addition ofthose components necessary to bring the composition within the requiredconcentration ranges. Ordinarily it is feasible to regenerate thepartially depleted bath by the addition of an iron compound such asferric chloride. Some decomposition of the EDTA I acid does occur, andit is usually desirable to regenerate the partially depleted bath bymeans of a mixture consisting of one mole of sodium salt of EDTA acidand from 2 to 10 moles of a water soluble iron salt such as ferricchloride. The triethanolamine also decomposes, especially at elevatedtemperature electrolysis, but no amine to iron molar ratios for bathregeneration were established. Because excess amine is conventionallyemployed initially, no replenishment of the amine is ordinarilynecessary.

EXAMPLE 16 Any of the plating baths may advantageously employ sodiumnaphthyl sulfonate or other modifying agent suitable for alkalineplating baths.

EXAMPLE 17 A plating bath similar to that of Example 5 was preparedmaking use of relatively large amounts of sodium carbonate, to whichadditional amounts of sodium hydroxide were added to raise and maintainthe pH above 12. The absorption of carbon dioxide from the air increasesthe difliculty of preserving the alkalinity at the required level. Theuse of relatively large ratios of sodium carbonate to sodium hydroxidein formulating the bath helps in providing a bath having a longer periodof usefulness under intermittent operating conditions.

EXAMPLE 18 A series of experiments were conducted to show that theelectroplating process of the present invention was operable when thevoltage between the electrodes was varied from 1 volt to 6 volts. Underusual operating conditions, satisfactory control of the current densitywill result in a potential in excess of 3 volts.

Any conventional type of electroplating cell may be utilized. Cells,generally called Hull cells and Haring cells, were utilized in certainof the examples described.

The examples heretofore given are merely for purposes of illustrationand are not intended to restrict the invention, which is defined in theappended claims.

The invention claimed is:

l. The process of electroplating iron onto an article which includes thestep of preparing a bath containing water, from 0.5 to 5 molar alkalihydroxide, from 0.5 to 5 molar organic amine, from 0.02 to 0.5 molarferric compound, and a salt of ethylenediaminetetraacetic acid in amolar ratio relative to the iron of from 0.5 to 1.5, the solution havinga pH greater than 11, making the article to be coated a cathode in anelectroplating cell, and passing an electric current through the cell ata current density of from 3 to 300 amperes per square foot of cathodesurface.

2. The process of claim 1 in which the bath is maintained at an elevatedtemperature.

3. The process of electroplating iron upon an article which includes thesteps of preparing a plating bath consisting essentially of 100 gramsper liter of sodium hydroxide, 154 grams per liter of triethanolamine,132

I grams per liter of the sodium salt of ethylenediaminetetraacetic acid,20 grams per liter of iron in the ferric state, water, said aqueoussolution having a pH greater than 11, causing said article to be acathode immersed in said plating bath, causing an electric current toflow in said solution between an iron anode and said cathode at acurrent density of from 3 to 300 amperes per square foot of cathodesurface, whereby an adherent, non-porous film of iron is plated ontosaid article.

4. The process of electroplating iron upon a cathode by the passage ofan electric current through a solution containing concentrations ofapproximately 0.36 molar iron, 0.33 molar salt ofethylenediaminetetraacetic acid, and 3.5 molar base comprising at least0.5 molar organic amine and at least 0.5 molar'inorganic base andaffording a pH greater than 11.

5. A plating solution adapted for the deposition of iron upon anarticle, said solution consisting essentially of water, the ferric saltof ethylenediaminetetraacetic acid in a concentration of from 0.02 to0.4 molar, and a mix- 10 ture of an organic amine and an inorganic baseeach in a concentration greater than 0.5 molar and aifording thesolution a pH in excess of 11.

6. A concentrate for alkaline plating of iron, consisting essentially ofa mixture of approximately seven moles of an alkali hydroxide, threemoles of an alkanolamine, from 0.8 to 1.2 moles of a salt ofethylenediaminetetraacetic acid, and an iron compound providing oneatomic weight of iron.

References Cited in the file of this patent UNITED STATES PATENTS1,729,607 Bezzenberger Oct. 1, 1929 FOREIGN PATENTS 731,102 Germany Feb.3, 1943 OTHER REFERENCES Senderoff, Metal Finishing, vol. 48 (Sept.1950), pp.

Watts et al., Transactions American Electrochemical Society, vol. 25(1914), pp. 529-536.

1. THE PROCESS OF ELECTROPLATIN IRON ONTO AN ARTICLE WHICH INCLUDES THESTEP OF PREPARING A BATH CONTAINING WATER, FROM 0.05 TO 5 MOLAR ALKALIHYDROXIDE, FROM 0.5 TO 5 MOLAR ORGANIC AMINE, FROM 0.02 TO 0.05 MOLARFERRIC COMPOUND, AND SALT OF ETHYLENEDIAMINETETRAACETIC ACID IN A MOLARRATION RELATIVE TO THE IRON OF FROM 0.5 TO 1.5, THE SOLUTION HAVING A PHGREATER THAN 11, MAKING THE ARTICLE TO THE COATED A CATHODE IN ANELECTROPLATING CELL, AND PASSING AN ELECTRIC CURRENT THROUGH THE CELL ATA CURRENT DENSITY OF FROM 3 TO 300 AMPERES PER SQUARE FOOT OF CATHODESURFACE.